1. Field of the Invention
The present invention relates to a surface acoustic wave device having a quartz substrate, and more particularly, to a surface acoustic wave device which is adapted to generate a fundamental mode of a leaky surface acoustic wave.
2. Description of the Related Art
A surface acoustic wave (SAW) device has been widely used as, for example, a bandpass filter in mobile communications equipment. Generally, a surface acoustic wave device has a piezoelectric substrate and at least one interdigital transducer (IDT) including at least one pair of comb-shaped electrodes disposed on the piezoelectric substrate. The substrate material of the surface acoustic wave device may be a piezoelectric single crystal such as LiNbO3, LiTaO3, or quartz, and piezoelectric ceramics such as PZT piezoelectric ceramics.
In order to achieve wide band characteristics in the surface acoustic wave device, a piezoelectric material must have a large electromechanical coupling coefficient. In some specific applications, the piezoelectric material is further required to have a good temperature characteristic, that is, the frequency shift of its filter characteristics caused by a change of temperature must be small.
Quartz is known as a substrate material having a small temperature coefficient of delay (TCD) among the aforementioned piezoelectric materials. Therefore, there have been proposed various types of surface acoustic wave devices each using a quartz substrate.
However, if Rayleigh waves are excited on a quartz substrate, there arises a problem that the electromechanical coupling coefficient is undesirably small. For example, the electromechanical coupling coefficient ks is at most 3.7% when a quartz substrate having an excellent TCD is used.
In addition, there arises a problem that the Rayleigh waves excited on the quartz substrate have such a small sound SAW that the surface acoustic wave device including the quartz substrate cannot be used in a high frequency device. Although it is known that a leaky surface acoustic wave having a relatively large sound velocity could be excited on a quartz substrate, the amount of decay of the leaky surface acoustic wave due to propagation is so great that the leaky surface acoustic wave is regarded as difficult to use.
For the aforementioned reasons, it is very difficult to realize a surface acoustic wave device which can operate at high frequencies and have a large electromechanical coupling coefficient and a small TCD.
For example, Japanese Laid-Open Patent Publication No. 61-222312 discloses a surface acoustic wave device in which a piezoelectric thin film is disposed on a quartz substrate and an IDT electrode is disposed on the piezoelectric thin film. This Japanese Patent publication discloses that a surface acoustic wave having an acoustic velocity about 1.7 times as large as that of a normal Rayleigh wave can be achieved by using a ST-cut quartz substrate and forming an electrode such that the propagation direction of the surface acoustic wave is perpendicular to the x-propagation direction. However, the surface acoustic wave that is regarded as available in this prior art is actually a combination of a surface shimming bulk wave (SSBW) and surface transverse wave (STW) which are close in frequency. Thus, it is difficult to be used in a surface acoustic wave resonator or the like.
For the foregoing reasons, there is a need for a surface acoustic wave device which has a small TCD and a large electromechanical coupling coefficient and is suitable for use at high frequencies.
The preferred embodiments of the present invention solves the aforementioned problems experienced in conventional devices. According to one preferred embodiment of the present invention, a surface acoustic wave device comprises a quartz substrate, a piezoelectric thin film disposed on the quartz substrate and an interdigital electrode disposed in contact with the piezoelectric thin film. The quartz substrate has an angle xcfx86 at the Euler angle (0, xcfx86, xcex8) which is selected such that the quartz substrate has a negative temperature coefficient of delay at a predetermined propagation direction xcex8. The piezoelectric thin film has a positive temperature coefficient of delay and a thickness H which is selected such that a fundamental mode of a leaky surface acoustic wave is excited on the quartz substrate, and the surface acoustic wave device operates using the fundamental mode of the leaky surface acoustic wave.
A normalized film thickness H/xcex obtained by dividing the thickness H of the piezoelectric thin film by a wavelength xcex of the leaky surface acoustic wave to be excited is preferably within the range of about 0.01 to about 0.15.
The angle xcfx86 is preferably in the range of about 119xc2x0 to about 167xc2x0 and more preferably in the range of 119xc2x0 to 138xc2x0.
The propagation direction xcex8 is preferably in the range of about 85xc2x0 to about 95xc2x0.
The piezoelectric thin film is preferably made of a material selected from the group consisting of ZnO, AlN, Ta2O5, or CdS.
The interdigital electrode may be disposed between the piezoelectric thin film and the quartz substrate. Further, the surface acoustic wave device may include a ground electrode disposed on the piezoelectric thin film.
In the surface acoustic wave device according to the preferred embodiments of the present invention, a quartz substrate has an angle xcfx86 selected such that a temperature coefficient of delay (TCD) becomes a negative value at the predetermined propagation direction while a piezoelectric thin film has a positive temperature coefficient of delay. Also, a piezoelectric thin film having a positive temperature coefficient of delay is provided on the quartz substrate. Therefore, the combination of the unique features of the quartz substrate and the piezoelectric thin film, including the respective negative temperature coefficient of delay and positive temperature coefficient of delay define a reduced value of TCD, thereby forming a composite substrate having a very small TCD.
Further, the thickness of the piezoelectric thin film is adapted to generate a fundamental mode of a leaky surface acoustic wave. Therefore, it is possible to provide a surface acoustic wave device that operates at the fundamental mode of a leaky surface acoustic wave and has a very small TCD.
In addition, in the case where a normalized film thickness of the piezoelectric thin film is set within the above specified preferred range, it is possible to form a surface acoustic wave device having not only a very small TCD, but a high electromechanical coupling coefficient.
Moreover, since the Euler angle of the quartz substrate is set at about (0, 119xc2x0 to 167xc2x0, 85xc2x0 to 95xc2x0), it is possible to reliably provide a surface acoustic wave device operating at the fundamental mode of a leaky surface acoustic wave and having a very small temperature coefficient and a large electromechanical coupling coefficient.